Warning: The following contains SPOILERS for The Flash season 8, episode 3, "Armageddon, Part 3."
One scene in Arrowverse crossover involving the sorcerer Felix Faust occurred at some point following Crisis on Infinite Earths. The scant details in the brief exchange between The Flash and Black Lightning suggest that the soul-stealing sorcerer may have found a way to take control of the World's Finest heroes. This apparently led to the development of the Injustice Protocols, which could be enacted to permanently depower any hero who was determined to be beyond saving.
The development of an delay problems in the Arrowverse, killing the hope of an increased number of crossover events now that all the CW heroes were living on the same Earth.
The Flash season 8 tried to rectify this with a five-part crossover event dubbed Armageddon, which promised an extensive number of cameos by heroes across the Arrowverse. However, the third chapter of Armageddon also hinted at an unseen crossover event that occurred sometime within the past year. In the opening scene of "Armageddon, Part 3," a panicked Barry Allen met with Jefferson Pierce at the Hall of Justice and insisted on activating the Injustice Protocols, which are unrelated to the Injustice game and comics. The Injustice Protocols were reportedly designed to protect the world in case one of the major heroes went rogue. When Jefferson proved reluctant, Barry reminded him that "we all designed this plan together after Faust." This seems to be a reference to the warlock Felix Faust, who was a frequent foe of the Justice League in the comics.
Felix Faust has made only a single canon appearance in the Arrowerse, having been the antagonist of the Cisco Ramon's villain binder, which listed numerous supervillains from Earth-1 and Earth-38 who had been reborn on Earth-Prime. The new Felix Faust might also have been from another Earth, as Supergirl faced a different version of Felix Faust on Earth-38 in the Supergirl tie-in novel "Supergirl: Master of Illusion."
In any case, any version of Felix Faust would be a formidable foe, as he frequently switched souls between bodies and possessed others' bodies in the comics. It seems likely that the Arrowverse version of Faust on Earth-Prime did something similar, leading to the development of the Injustice Protocols mentioned in The Flash. Logically, this means that the heroes of the Arrowverse Justice League must have teamed up at least once in the past year and that all of the major heroes have made arrangements with one another, such as Supergirl entrusting Batwoman with the last piece of Kryptonite on Earth-Prime, in case the worst should happen again.